You have to be truthful and input your correct marital status on your application for naturalization: "married". Otherwise you risk having your application denied based on "fraud" and or "lack of good moral character."

Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.

My concern with the posted question is that it sounds like the person has a lot of official paperwork that says "single" (including tax forms) but the person is really married. So the issues may be more complex than the person thinks. Also, I have run into a number of complex cases involving people from the Philippines who were confused about their marital status and put the wrong thing on various visa forms. So I do think this person needs to talk to a lawyer and go over every piece of paper with the lawyer. That said, you are absolutely correct that the person must tell the truth if the person files an N400.

If you are married, you are married and that it what you need to disclose.

The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.